Add an httpreq endpoint for lego and Traefik

jarld can now answer lego's httpreq DNS provider
(POST /api/v1/acme/httpreq/present and /cleanup), so lego and Traefik
publish DNS-01 challenges with only environment variables, without the
jarl CLI. lego's username and API token arrive as HTTP Basic auth,
accepted on these routes only.

The endpoint is off by default (acme.httpreq.enabled) and HTTPS-only by
default: jarld's own TLS, or a trusted proxy that says so with
X-Forwarded-Proto or Forwarded (the last value counts). Plain HTTP can be
allowed from loopback only or everywhere. GET /api/v1/acme/httpreq shows
how jarld sees a request, to check a proxy setup.

jarl acme and the endpoint share their code in internal/acme. Documented
in docs/acme.md, configuration.md, api.md and permissions.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/development.md

@@ -23,6 +23,7 @@ internal/
   stats/              query statistics: statistics-channel poller, response-log tailer, collector, separate SQLite store
   web/static/         the web UI: plain ES modules, no build step, embedded with go:embed
   api/, client/       JSON types and the Go client shared by server and CLI
+  acme/               ACME DNS-01 challenge records, shared by jarl acme and the httpreq endpoint
   config/             jarld's YAML configuration
 deploy/               systemd unit, sysusers.d and tmpfiles.d files
 contrib/acme.sh/      acme.sh DNS API hook